When membranes suitable for filtering out bacteria and smaller entities were developed by Zsigmondy and others between World Wars I and II, all such membranes were known as ultrafiltration membranes. At the present state of the art the definition of "ultrafiltration membranes" has been narrowed and additional loosely defined names introduced:
In this specification and the appended claims "nanofiltration membranes" are defined as membranes which reject less than about 50% of sodium chloride when challenged at about room temperature and at about 225 pounds per square inch trans-membrane pressure difference with a solution in water containing about 2000 milligrams of sodium chloride per liter and which in a separate test reject more than about 50% of magnesium sulfate when challenged at about room temperature and at about 225 pounds per square inch trans-membrane pressure difference with a solution in water containing about 2000 milligrams of magnesium sulfate per liter. Such membranes typically reject 90% or more of albumin but may or may not reject a substantial fraction of disaccharides.